Hypertrophic cardiomyopathy is a primary myocardial disease characterized by a wide spectrum of different clinical and morphologic manifestations, including patients with and without obstruction to left ventricular outflow. To characterize the development of subaortic obstruction during childhood and to define morphologic determinants of this phenomenon, we studied 26 consecutive children who presented with nonobstructive hypertrophic cardiomyopathy at their first evaluation. During a mean follow-up of six years, 7 patients (27%) developed dynamic obstruction to left ventricular outflow as identified by the appearance of systolic anterior motion of the mitral valve on serial echocardiographic examinations. Both groups of patients (with and without development of obstruction) showed progression of left ventricular hypertrophy during follow-up. While patients who remained nonobstructive showed homogeneous progression of all left ventricular segments, the 7 patients with development of obstruction had selective increase in thickness of the basal anterior septum with a consequent reduction of the left ventricular outflow tract. These findings indicate the subaortic obstruction may appear in the hypertrophic cardiomyopathy during childhood. This phenomenon is associated with and is probably a consequence of narrowing of the left ventricular outflow tract secondary to selective progression of hypertrophy in the basal anterior ventricular septum.